In recent years, an ultrafine fiber having a diameter of a nano meter order has been attracting attention, and is expected to be utilizable in various fields of battery●information, environment●energy, medical care (e.g., biocompatible material etc.)●welfare.
As a material for forming such ultrafine fiber, a wide variety of materials such as organic polymers (e.g., nylon and the like), inorganic substances (e.g., TiO2, SiO2 and the like), organism-derived polymers (e.g., cellulose, collagen and the like), and the like have been considered.
As a technique for spinning an ultrafine fiber having a diameter of a nano meter order, melt blow method, composite melt spinning method, electrospinning method and the like are known. Particularly, electrospinning method is attracting attention as a method capable of fibrosis of materials that could not be handled heretofore. For example, many medical polymers such as polylactic acid and the like, and water-soluble polymers such as polyvinyl alcohol and the like have been investigated in addition to the aforementioned organism-derived polymers such as cellulose, collagen and the like (patent documents 1-8, non-patent document 1).
On the other hand, as biocompatible materials such as cell culture scaffold material and the like, use of organism-derived materials (particularly, gelatin derived from bovine etc.) has been avoided in recent years due to safety problems, and production using non organism-derived materials (e.g., synthetic polymer etc.) has been desired.
Biocompatible materials such as cell culture scaffold material and the like require use of an organic solvent such as ethanol and the like for a sterilization treatment. When the above-mentioned ultrafine fiber is applied to the biocompatible materials, the fiber also needs to have resistance to organic solvents. In the above-mentioned patent documents and non-patent document, a method including crosslinking polymers by a crosslinking agent and the like are used as a means for improving durability of a fiber. However, when the kind of the polymer is different, crosslinking conditions vary, and a complicated treatment such as UV irradiation, hydrogen chloride gas treatment and the like is sometimes required (e.g., patent documents 3, 7 and non-patent document 1). Therefore, a method capable of producing a fiber having organic solvent resistance by a simple treatment alone (e.g., heat treatment alone, preferably, heat treatment at low temperature for short time alone) is desired.